Cyclosiloxanes are prepared by a number of processes. Hydrolysis of dialkyldichlorosilanes, RR′SiCl2, comprises the original process and is practiced on the industrial scale. Hydrolysis often yields complex mixtures of liner and cyclic siloxanes, with cyclotetrasiloxanes formed in the highest proportion and with little formation of cyclotrisiloxane. (see W. Noll, “Chemistry and Technology of Silicones”, Acad. Press, 1968).
An alternate process for the preparation of cyclosiloxanes is acid or base-catalyzed depolymerization of polysiloxanes. (see Kostas, U.S. Pat. No. 5,491,249) This is often employed for the production of some cyclotrisiloxanes and cyclotetrasiloxanes from homopolymers of dialkylsiloxanes, although formation of significant amounts of cyclotrisiloxanes generally requires high temperatures. Mixed cyclosiloxanes, those containing two or more different siloxane repeating units, can be produced in this manner, although isolation of the individual components from the complex mixture can be difficult depending upon the substituents on the repeating units of the cyclosiloxanes and the proportions of these repeating units in the depolymerizing copolymer. (see Buese et al., U.S. Pat. No. 5,247,116)
Processes to prepare specific cyclosiloxanes, particularly cyclotrisiloxanes, include coupling of dichlorosilanes, R1R2SiCl2, or dichlorosiloxanes, Cl(R1R2SiO)x(SiR3R4O)ySiR5R6Cl, with silane diols, R7R8Si(OH)2 or siloxane diols, HO(R7R8SiO)x(SiR9R10O)yH (where R1, R2, R3, R4, R5, R6, R7, R8, R9, and R10 are the same or different and x≧0, y≧0, and x+y≧1). (see Yuzhelevskii et al., Zhurnal Obshchei Khimii (1972), 42, (9), 2006-10). The disadvantages of this process include: susceptibility to side products due to reaction with water impurities common in the diols; competing condensation of the diols to larger siloxane diols; corrosiveness to reaction vessels; and potential siloxane redistribution catalyzed by the liberated HCl.
Lewis acid catalyzed processes have been examined that can form large quantities of cyclosiloxanes, particularly cyclotrisiloxanes have been examined. Elimination reactions involving a hydridosiloxane, for example 1,1,3,3,5,5,7,7-octamethyltetrasiloxane, using an aromatic substituted metal halide, permit the formation of a cyclotrisiloxane, for example hexamethylcyclotrisiloxane, with the loss of a dialkylsilane, for example dimethylsilane, (CH3)2SiH2, as the main product, although significant amounts of linear dimethylsiloxane polymers, cyclotetrasiloxanes and other cyclosiloxanes can form depending upon the reaction conditions and time between introduction and quenching of the catalyst. (see Rubinsztajn et al., U.S. Pat. No. 7,148,370). Condensation reactions involving a dialkoxysilane, (H5C6)2Si(OCH3)2 and a dihydrosiloxane 1,1,3,3-tetramethyldisiloxane, using an aromatic substituted metal halide, permit the formation of 1,1-diphenyl-3,3,5,5-tetraamethylcyclotrisiloxane with significant amounts of unidentified linear siloxane oligomers. (see Rubinsztajn et al., US Patent Application Publication No. 2004/0127668) Dehydrocondensation of 0.1 M diphenylsilanediol with an equimolar amount of 1,1,3,3-tetramethyldisiloxane or 1,1,3,3,5,5-hexamethyltrisiloxane using 0.25 M ZnCl2 in DMF gave cyclotrisiloxane (˜30%) and cyclotetrasiloxanes (˜50%), respectively, accompanied by a viscous oil. (see Chrusciel et al., Polish Journal of Chemistry (1983), 57, 121-7). Hence there remains a need for a process to form mixed cyclosiloxanes, including cyclotrisiloxanes, in high yield and in an easily isolated manner.